As the conflict in the Gaza Strip and Israel moves into its third week, the impact of the fighting on Palestinian and Israeli children has become a heart-breaking signature of the conflict. Reports from Gaza relay stories of shells destroying civilian homes, killing children sheltering within; of tank fire killing a 5-month-old baby; of a strike on a beach killing four young boys who had been kicking around a soccer ball. In Israel, parents hear the first wail of air-raid sirens, grab their frightened children and run for bomb shelters.

After two weeks of aerial attacks by the Israel Defense Forces (IDF) against the Palestinian militant group Hamas in the densely populated Gaza Strip — followed by a ground offensive that began last week — the statistics are just as grim as the news reports. UNICEF’s communications chief in Jerusalem, Catherine Weibel, says that according to U.N.’s figures at least 121 Palestinian children under the age of 18 have been killed since the conflict started on July 8, making up a full one-third of Palestinian civilian casualties. Between July 20 to July 21 alone, she says, “there were at least 28 children killed in Gaza.”

The conflict, which was, in part, precipitated by the killing of children — first the murder of three Israeli teens in late June and then one Palestinian teen earlier this month — has so far claimed the lives of a further 479 Palestinians and 27 Israelis, according to the latest U.N. figures. But it’s perhaps the escalating toll on innocent children that has drawn the greatest concern from human rights organizations, world leaders and critics of both Hamas and Israel. In a press conference on July 22, U.N. Secretary-General Ban Ki-moon called for a cease-fire, saying that “too many Palestinian and Israeli mothers are burying their children.” The day before, U.S. President Barack Obama said in a press conference that there was an urgent need to “stop the deaths of innocent civilians.”

In response to the outcry over the loss of children’s lives — much of it directed toward Israel — the Israeli military has said that it has gone to extensive lengths to prevent civilian deaths, while psychologists in Israel point out that the conflict has taken a toll on Israeli as well as Palestinian children.

Lieut. Colonel Peter Lerner, a spokesman for the IDF, tells TIME that Israel has made efforts to “encourage people to leave areas that were potential combat zones” by releasing leaflets, sending texts and making phone calls before the IDF launched its ground offensive. Lerner says that civilians are never the targets. “Some of the targets have been civilian homes that have been utilized for command and control positions by terrorists,” he says.

Bill Van Esveld, a senior researcher at Human Rights Watch who is based in Israel, the West Bank and Gaza, tells TIME that “given the fact that more than 50% of Gaza’s population is under 18 years old,” any attacks on the region’s densely packed residential areas are likely “going to harm civilians [and] chances are, given that population ratio, going to be killing kids.”

Given the large, young population in Gaza, experts say that many children are likely suffering from symptoms of trauma, even if they haven’t been physically injured. The U.N. Office for the Coordination of Humanitarian Affairs (OCHA) says that “at least 72,390 children require direct and specialized psychosocial support (PSS) on the basis of families who have experienced death, injury or loss of home over the past 10 days.” OCHA also expects that number to soar.

An Israeli mother and her son run to a bomb shelter as the siren goes off in Ashkelon, Israel, on July 14, 2014.

Ilia Yefimovich—Getty Images

So far no Israeli children have been killed in the conflict and there are no reports of injuries. But Israeli activists and researchers report that the conflict has traumatized children on both sides of the walls and fences that separate Gaza from Israel. Hamas has fired 2,000 missiles into Israel, according to the IDF, since the conflict began. Says clinical psychologist Yotam Dagan, the international cooperation director at Natal, an Israeli nongovernmental organization that treats victims of trauma related to war and terror: “It’s not just an Israeli problem and it’s not just a Palestinian problem — children are children are children, everywhere. Whenever hostilities break, you know the fire starts to blow [and] children are the first and the most likely to be affected by the situation.”

“When we talk about psychological trauma it’s like an invisible bullet,” Dagan tells TIME, “nobody sees it, but this experience of being near death or being nearly killed or exposed to explosions and rockets falling — or even the fear — it’s like an invisible bullet that goes through your soul, through your mind.”

Irwin Mansdorf, an Israeli psychologist and fellow at the Jerusalem Center for Public Affairs, says that Israeli children exposed to attacks by Hamas are susceptible to “real clinical symptoms as a result of real, distressing events.” According to both psychologists, the symptoms of trauma in children, much like in adults, can include nightmares, flashbacks, detachment, anxiety, depression or acting out and regression.

Some Israeli children are also dealing with being displaced from their homes. Roni Taronski, 12, is seeking refuge — along with her mother, grandmother and aunt — in a boarding school about 11 miles (18 km) southeast from her home in Kibbutz Mefalsim, which is only a mile from the Gaza border and has become a target of Hamas rocket fire. “It’s not that we don’t want to go, everybody wants to go home,” she tells TIME. “We’re not allowed. Our houses are [like] a military base right now.” When asked how she’s feeling so far away from home, Roni says she’s homesick. She then adds, “I’m really scared.”

In Gaza, Samira Attar, a 13-year-old girl from the Beit Lahia neighborhood, is among the 100,000 Palestinians who are now displaced and seeking refuge in an U.N. Relief and Works Agency (UNRWA) school, where her mother Souad, says the sound of shelling keeps the family awake at night. “I hate this school,” Samira tells TIME. “I want to play with my cousins and sisters and live normally like others.”

Twelve-year-old Shahd Majed lies on a bed in Gaza City’s Shifa Hospital, her leg injured by shrapnel from a shell or bomb that exploded near her house on July 21. “I hate the war,” she says. “I want it to end please. I want to return to normal life. Please let us live.”

— With reporting by Hazem Balousha / Gaza

Correction: An earlier version of this story drew a premature conclusion about the source of the Israeli attack that killed four boys in Gaza. Israel is still investigating the strike.

Israel's Ground Invasion of Gaza Continues

A Palestinian Christian man from Gaza tries to push the coffin of Jalila Ayad in her grave during her funeral on the small and overcrowded cemetery of the St. Porfirius church in Gaza City, July 27, 2014.

Oliver Weiken—EPA

Palestinian men gather things they found in the rubble of destroyed buildings on July 27, 2014 in the Shejaiya residential district of Gaza City as families returned to find their homes ground into rubble by relentless Israeli tank fire and air strikes.

Marco Longari—AFP/Getty Images

Palestinians recover the body of a man killed when his home was hit the previous night by Israeli fire in the northern district of Beit Hanoun in the Gaza Strip during an humanitarian truce, on July 26, 2014.

Palestinian women react amid the destruction in the northern district of Beit Hanoun in the Gaza Strip during an humanitarian truce on July 26, 2014.

Marco Longari—AFP/Getty Images

A general view of destroyed buildings after Israeli attacks in a part of the Shuja'iyya neighborhood in east Gaza City, July 26, 2014.

Oliver Weiken—EPA

Palestinian girls peers from inside a UN school in Jabalia, north Gaza Strip, on July 25, 2014, where they found shelter after escaping from their home.

Marco Longari—AFP/Getty Images

Smoke from an Israeli strike rises over the Gaza Strip, July 25, 2014.

Majdi Fathi—ZumaPress

Israeli APCs drive near the Israeli border with Gaza as they come out of the Gaza Strip July 25, 2014.

Nir Elias—Reuters

Mortar cases are piled at a military staging area near the border with the Gaza Strip, July 24, 2014.

Nir Elias—Reuters

A Palestinian man holds a girl injured during shelling at a U.N.-run school sheltering Palestinians, at a hospital in the northern Gaza Strip on July 24, 2014.

Alessio Romenzi for TIME

Blood stains of displaced Palestinians are seen inside the UNRWA school in Beit Hanoun after it had been hit, Gaza Strip, July 24, 2014.

Alessio Romenzi for TIME

A Palestinian woman stands in front of buildings damaged by Israeli bombardment in the Jabalia district of the northern Gaza Strip on July 24, 2014.

Marco Longari—AFP/Getty Images

Relatives hold the bodies of Palestinian children Hadi Abdel Nabi, 3, and one and a-half-year-old Abdel Rahman Abdel Nabi, at the cemetery in Jebaliya refugee camp in the northern Gaza Strip, during their funeral, July 24, 2014.

Lefteris Pitarakis—AP

A young Palestinian girl who got injured when a UN school for refugees was allegedly hit by a Israeli tank shells, lies on a hospital bed in the emergency room of Kamal Adwan hospital in Beit Lahiya, Gaza Strip, July 24, 2014.

Oliver Weiken—EPA

Displaced Palestinians from Beit Hanoun sleep inside the UNRWA school in Jabalia, July 23, 2014.

Alessio Romenzi for TIME

Wounded Israeli soldiers are brought to a helicopter on July 23, 2014 near Kafar Aza, Israel.

Andrew Burton—Getty Images

Soldiers carry the coffin of Sergeant Max Steinberg during his funeral on July 23, 2014 in Jerusalem.

Ilia Yefimovich—Getty Images

Comrades of slain Sgt. Max Steinberg are comforting each other at Mt. Herzl cemetery, Jerusalem, July 23,2014.

Omer Messinger—NurPhoto//REX USA

Smoke pours out of the ground due to an alleged campaign by the Israeli military to fill tunnels originating in Gaza with smoke to discover entrances into Israel on July 23, 2014 near Sderot, Israel.

Andrew Burton—Getty Images

Palestinians take cover as warning Israeli air strikes are fired at a nearby building in Gaza City, July 22, 2014.

A Palestinian boy, who medics said was wounded by Israeli shelling, receives treatment at al-Shifa hospital in Gaza City July 20, 2014.

Ali Jadallah—APA/Landov

A doctor cries while standing next to a table with the bodies of four dead children in overflowing morgue of the Shifa hospital in Gaza City, July 20, 2014.

Oliver Weiken—EPA

A Palestinian woman wearing clothes stained with the blood of other relatives, who medics said were wounded in Israeli shelling, cries at a hospital in Gaza City July 20, 2014.

Mohammed Salemm—Reuters

Palestinians, who medics said were wounded during heavy Israeli shelling, sit at a hospital in Gaza City July 20, 2014.

Mohammed Salem—Reuters

A Palestinian woman, who medics said was wounded by Israeli shelling, receives treatment at al-Shifa hospital in Gaza City, July 20, 2014.

APAimages/Rex

Two Palestinian men carry a white flag as they flee their homes during a brief period of ceasefire requested by local rescue forces to retrieve dead and wounded from the Shuja'iyya neighbourhood in east Gaza City, July 20, 2014.

Oliver Weiken—EPA

A Palestinian girl sits in a minibus after fleeing her family's house during heavy Israeli shelling, in Gaza City July 20, 2014.

Suhaib Salem—Reuters

A Palestinian woman, who fled her house following an Israeli ground offensive, stays at a United Nations-run school in Rafah in the southern Gaza Strip July 20, 2014.

Ibraheem Abu Mustafa—Reuters

Israeli soldiers fire their weapons during the funeral of their comrade Bnaya Rubel in Holon, near Tel Aviv July 20, 2014.

Nir Elias—Reuters

Israeli soldiers carry the coffin of killed Israeli soldier Amotz Greenberg during the military funeral ceremony in the cemetery of Hod Hasharon, Israel, 20 July 2014.

Abir Sultan—EPA

Smoke rises during what witnesses said were heavy Israeli shelling at the Shejaia neighbourhood in Gaza City July 20, 2014.

Mohammed Salem—Reuters

A Palestinian family who fled their homes is en route to seek shelter in a UN school in Khan Younis, Gaza Strip on July 18, 2014.

Oliver Weiken—EPA

A Palestinian child along with her family, who fled their home during an Israeli ground offensive, take refuge at a UN school in Gaza City with other families, on July 19, 2014.

Mahmud Hams—AFP/Getty Images

Palestinians, who fled their houses following an Israeli ground offensive, at a UN school in Rafah in the southern Gaza Strip, July 19, 2014.

Eyad Al Baba—Apaimages/Polaris

Israeli tanks maneuver outside the Gaza Strip on July 18, 2014.

Ronen Zvulun—Reuters

Israeli soldiers put on their gear on the side of a road across from the Gaza Strip on July 18, 2014.